For large areas of chainmail, I've found this to be a workable method. The technique was developed in concert with kitty. It's not perfect, and it's definitely not ideal for loose chaimail that isn't over a dark background of some sort, such as coifs, but it does well for large areas like hauberks.

Step two: shade this - you'll want quite a lot of contrast here, since this layer will be under everything else.

Step 3: use a round brush on a new layer to paint on a sequence of dots which will form the basis of your mail, in a shadow colour. These should be reasonably close together in areas which face the viewer directly, with gaps between them maybe 1/4 the diameter of the circles, which I'd usually put at between 15 and 19px in diameter at my working scale of around 1500 px square - the smaller the rings, the better the quality of the mail, generally. This isn't a set distance, and you'll be able to tell at the end if they're too far apart. As they curve around to form you're illustrating, they should get closer together and overlap. Each subsequent row of rings will be offset against the previous one, as you see.

Step 4: (this isn't directly pictured, since I've already shaded this stage on the images) use a relatively hard-edged tool to paint on the highlights, again on a new layer. They go over roughly 2 thirds of the edge of each of your dark spots. The direction of the highlights depends on the light direction, but it'll often be perfectly acceptable to have the highlights on different sides of the rings. In GIMP, you can accomplish this stage procedurally by using the feature by which whatever's on the clipboard can be used as a paintbrush - draw one out, copy it, then you can stamp it over each one very quickly.

Step 5: lock the transparency on each layer, and shade them, following rules of shading as normal.

Step 6: Countershading - the edges around the darkest areas will usually have bright highlights - these tend to extend in along the lines of rings.

It's a little vague, and you should definitely check out a good deal of reference material, but hopefully this will boost you on your way.

As we did mention this a while ago, I'm now adding to this tutorial an explanation of my own method, which was developed after TSI's initial research. Fetch your graphic tablet, here goes Lordbob's Chainmail Recipe !

IntroductionNote that this tutorial is based on Photoshop, but is very likely to work just as well with Gimp. It's also meant for advanced users as it doesn't explain each and every single manipulation, especially the subtleties of brush customization. If you don't know about this, you should learn it from another source before you worry about chainmail.
The assumption underlying this technique is that, when looking at chainmail from a distance, our brain does not precisely analyse the layout of every single ring. I'd even go as far as supposing that we suck at that : should you grab a piece of real chainmail and attempt to identify individual rings, you'll be rewarded with a headache in no time.
What our brain does instead is focus on a patterns of light reflections, shadows and shapes which it eventually recognizes as "chainmail". In other words, you don't have to painfully detail your chainmail. It'll actually be even more convincing if there's detail missing, since our brain fills in the gaps without questioning, whereas it'll easily spot a pattern that looks wrong (something which is very likely to occur if you go for the full detail).

Basic tools
It's best if you do this with 3 different layers at least :
- top layer : rings lights
- middle layer : rings darks
- bottom layer : chainmail flats
For optimal comfort, both "rings" layers should use the bottom layer as a clipping mask., so that your rings won't overlap other surfaces.
This technique relies on a custom brush that emulates the shapes of the lights and darks we perceive when looking at chainmail. I'm not claimings it's the best shape there is: simply it works well enough for my needs. If this one doesn't satisfy you, do not hesitate to come up with one of your own doing : Gimp and Photoshop are very flexible when it comes to custom brushes.

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Step 1: flats
Your chainmail should start as a flat surface (guessed where ? Yes, in the chainmail flats layer) with a colour choosen accordingly to whatever metal your chainmail's made of (here, dark gray for steel). Pick a luminosity that corresponds to your final chainmail's average value.

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Step 2: underlying material
Chainmail isn't completely opaque. In places, depending on the angle at which you look at it, you will get a glimpse of whatever's underneath: leather, cloth, bare skin, steel... An upwards angle is when you'll most see the underlying surface, whereas a low, raking angle is when you'll see only the rings.
In order to reflect this, we will shade our flat layer in order to add not only volume but also slight touches of the underlying material wherever it should be visible.

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Step 3: ring darks
Moving to the rings darks layer, we will now switch to our custom rings brush. Pick a colour that represents your chainmail's darkest shadows (here, pure black) and apply brush strokes that follow the pattern of your mail. With actual chainmail, most of the time you'll perceive "lines" that correspond to the alignment of rings : this is the direction your strokes should follow. Note that these lines should also reflect the volume of whatever your chainmail is covering - just doing straight strokes won't work (check some reference if you're unsure).

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Step 4: ring lights
Moving to the rings lights layer, we will keep our custom brush and optionnaly make its opacity pressure-sensitive (though this is in no way compulsory : you can apply flat lights and rework them later with the eraser, or a mask - use whichever works best for you). Pick a colour that represent's your chainmail lightest lights (here pure white). Keep in mind that the luminosity of said lights should depend of your picture's overall lighting. You can now apply your lights with brush strokes that follow the pattern of your darks, and vary their opacity and/or luminosity according to your overall lighting.

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For further practice
Once you've mastered these steps, you'll have a technique that is suitable for small, simple patches of chainmail. When upgrading to large chainmail display with many different angles of view (such as a full hauberk with no other garment), you should be prepared to use different brushes for different areas of a single surface. Keep in mind that simple shapes work better, whereas highly detailed ones imply complex -if not [censored]- transitions from one brush to the other. Sometimes, it may happen that using different shapes for lights and darks produces a more lifelike result. As an example, below is an alternate brush that I would use with an angle of view perpendicular to the chainmail surface, though in a larger picture (small details tends to get lost when scaling down from working size to final size).
Note that these (relatively) simple steps can be adpated for other kinds of surfaces, such as saurian/drakish scales. The key is choosing your basic light & dark pattern shapes.